A patient's blood chemistry is often measured as a vital indicator of the patient's physiological condition and health. Traditionally, blood chemistry has been measured by extracting blood from the patient and then analyzing the extracted blood. Although this type of measurement is accurate, it is invasive and expensive, often requiring administration by trained medical personnel. Fortunately, various noninvasive interactions with the blood are also used to measure a patient's blood chemistry. For example, absorption or scattering of light by a part of a patient's body allows blood chemistry to be measured without extracting blood from the patient. Iontophoresis and other transcutaneous techniques that migrate analytes through the skin also show promise for noninvasive blood chemistry measurement.
A prior art noninvasive measurement method and apparatus are taught by Robinson et al., U.S. Pat. No. 4,975,581, which issued Dec. 4, 1990. Robinson et al. measure optical absorption by a patient's finger and then compare the similarity of the measurement to a data model constructed from measurements of biological fluids having known concentrations of analytes. The concentration of analytes in the finger is then determined from the comparison.
Another prior art noninvasive measurement system is taught by Clarke in U.S. Pat. No. 5,054,487 which issued Oct. 8, 1991. Clarke measures optical scattering to determine the concentration of analytes, such as glucose and cholesterol in a patient's blood, by measuring the ratio of the intensities of incident to the scattered light from a body part and comparing the measurements to predetermined values. Barnes et al. in U.S. Pat. No. 5,070,874, which issued Dec. 10, 1991 and Rosenthal et al. in U.S. Pat. No. 5,086,229, which issued Feb. 4, 1992, measure transmission of near infrared radiation through a body part and then analyze the measured data to determine glucose concentration.
Unfortunately, factors such as the patient's level of hydration and body temperature and the presence of bone, cartilage and collagen influence these optical and near infrared radiation measurements, but the factors do not correspond directly to the patient's blood chemistry. Since these factors are difficult to account for in data models and data analysis relied upon, they contribute errors to the blood chemistry measurements. As blood chemistry is a vital indicator of the physiological condition and health of a patient, it is important that any method or apparatus used to measure blood chemistry is accurate.